1. Field of the Invention:
This invention relates to a power transmission for use in vehicles provided with a continuously variable transmission (hereinafter called "CVT").
2. Description of the Prior Art:
A CVT is used for an excellent power transmission system which is to control continuously speed ratio e(=the output side rotational speed Nout/input side rotational speed Nin) to improve the specific fuel consumption of a vehicle. In a belt type CVT, a belt is trained over a pair of input side pulleys and a pair of output side pulleys, the input and output side pulleys are provided respectively with hydraulic cylinders to control press forces of the input and output side pulleys according to oil pressure supplied to the hydraulic cylinder and oil pressure in the hydraulic cylinder of one of the input and output side pulleys (usually the output side pulleys) provides line pressure which is controlled in relation to the transmission force of the belt by an electromagnetic relief valve to thereby avoid the slip of the belt while restraining the drive loss of an oil pump. However, since the hydraulic cylinder is rotated integrally with the pulley, a centrifugal force acts on a hydraulic medium in the hydraulic cylinder so that actual oil pressure becomes larger than a controllably intended value due to said centrifugal force. This increases the press force of the pulleys, causing the degradation of the transmission efficiency of the CVT and the lives of respective parts of the CVT. For prior corrective measures against such degradation are there a method of offsetting constructionally the centrifugal force itself of the hydraulic medium and a method of providing an oil reservoir in a rotary portion integral with the hydraulic cylinder and detecting an oil pressure value in the oil reservoir as a signal related to the centrifugal force through a Pitot tube to correct the line pressure through a pressure regualting valve on the basis of the detected value. In the former method, however, the offsetting of the produced centrifugal force is limited to about 50% and in the latter method problems are encountered in that the construction is complicated while power loss due to stirring is brought about. Both methods present obstacles against the practical use of prior corrective measures.